The invention relates to a process for treating dissolution residues generated in the recovery of fission products, especially fission molybdenum.
For the production or fission products, especially fission molybdenum, nuclear fuels comprising the elements uranium, silicium and aluminum are particularly suitable. Such nuclear fuels may consist of an alloy or these elements (for example, U.sub.3 SiAl) or of uraniumsilicide, which is enclosed in a cladding or aluminum or an aluminum alloy.
The nuclear fuel is shortly irradiated in a react or whereby fission products are generated from the 235.sub.U contained in the nuclear fuel. Among those fission products the radioactive isotope .sup.99 Mo is of particular interest because radioactive decay thereof with a radioactive half-time of exchange or 66 hours, results in the isotope .sup.99m Tc which is used in medical diagnostics and which itself has a half-time or exchange or 6 hours.
If irradiated nuclear fuels or this type, such as U.sub.3 SiAl or aluminum-cladded uraniumsilicide, are treated with an alkali hydroxide solution, the aluminum and part of the fission products are dissolved, but a large part of the fission products remain in the solution residue, particularly the main part of the fission molybdenum (depending on the particle size of the irradiated uraniumsilicide nuclear fuel between 75 and 85%) together with the uranium and the silicon).
It has been tried (K. A. Burill, R. J. Harrison: "Development of the .sup.99 Mo process at CRNL"; Proc. of a Tech. Com. Meeting by IAEA-TECDOC-515, p. 35) to dissolve nuclear fuels directly by nitric acid out of an irradiated U.sub.3 SiAl alloy, that is, without preceding treatment with an alkaline solution. But it has been found that, then, up to 50% of the molybdenum is lost by the formation of gel-like species of silica. It has further been tried (G. F. Vandegrift, J. D. Kwok, S. L. Marshall, D. R. Vissers, J. E. Matos: "Continuing Investigations of Technical Assessment of .sup.99 Mo production from LEV targets"; Proc. of a Tech. Com. Meeting by IAEA from the 13th to 16th Oct. 1987. ,Karlsruhe, IAEA-TECQOC-515, p. 115) to dissolve the dissolution residue, which was generated during treatment of (non-irradiated) nuclear fuels of this type by an alkaline solution, with an additional solution consisting of 30% concentrated hydrogen peroxide and caustic soda solution. In this respect it has been reported that, although the residue could be dissolved, substantial problems have been encountered by the release of excess hydrogen peroxide as a result of which it became impossible to fully separate the uranium during the following recovery process.
It is well known that the treatment of the dissolution residue with a solution of 30% concentrated hydrogen peroxide and caustic soda solution is a risky procedure since, in the process, in the presence of fission noble metals and oxidic corrosion products, hydrogen peroxide can react spontaneously resulting in an explosion.
EP 0 197 271 Al discloses a process for the manufacture of highly refined molybdenum or molybdenum oxide powder, wherein molybdenum or molybdenum oxide powder is treated with a watery, preferably 5 to 35% concentrated hydrogen peroxide solution, whereby a molybdenum-containing solution is formed. A strong mineral acid may be admixed to the hydrogen peroxide solution. The molybdenum-containing solution is contacted by a cation exchanger and then concentrated. The solid components of the concentrated solution can be reduced by hydrogen to molybdenum. The process does not utilize the dissolution residues which are generated in the recovery of fission products, particularly fission molybdenum.
From GB 2 229 312 A it is known to dissolve actimides or fission products with a solution which contains hydrogen peroxide and a complex-forming agent such as citrate. During dissolution the pH value is to be maintained above 5.5.
Finally, it is known from World Patent Index Latest, AM:86-071874/11 concerning J6102088A to dissolve cladding material of nuclear fuel with attached deposits of Mo and others in an alkaline solution of sodium hydroxide and hydrogen peroxide and subsequently with nitric acid.
It is the principal object of the present invention to safely and as much as possible totally dissolve the dissolution residues in a method of the type described above without encountering problems with the subsequent processing steps for the isolation of the fission products, particularly the molybdenum and the uranium.